1. Field of the Invention
The present invention relates to a semiconductor light emitting device, and more particularly, to a semiconductor light emitting device capable of maximizing an emission of light generated at a light emitting layer to outside and easily spreading a current to the light emitting layer, and a fabrication method thereof.
2. Description of the Related Art
Generally, a semiconductor is a direct transition type, and has been used to form a light emitting wavelength from a red region to a purple region and an ultraviolet ray region due to a high light emitting efficiency. As the understanding for a growing method and a structure of the semiconductor is increased, characteristics of a light emitting device, that is, a brightness, a driving voltage, or a static characteristic have been improved.
However, in spite of these efforts, a high output and a low driving voltage are much required, and a nitride semiconductor light emitting device that outputs a long wavelength (yellow and red) and a short wavelength (ultraviolet rays) has to be continuously researched. FIG. 1 shows a structure of a nitride semiconductor light emitting device in accordance with the conventional art.
As shown, the conventional nitride semiconductor light emitting device comprises: a sapphire substrate 10; an n-doped GaN layer 11 on the sapphire substrate 10; a light emitting layer 12; a p-GaN layer 13; a transparent electrode 14 formed on the p-GaN layer 13; a p-pad electrode 15 on the transparent electrode 14; and an n-pad electrode 16 formed on the n-GaN layer 11 exposed by vertically mesa-etching from the p-GaN layer 13 to a part of the n-GaN layer 11.
FIG. 2 is a view showing a structure of another nitride semiconductor light emitting device, that is, a Top-down electrode type semiconductor light emitting device in accordance with the conventional art. As shown, on a silicon carbide (SiC) substrate 20, an n-GaN layer 21, a light emitting layer 22, a p-GaN layer 23, and a transparent electrode 24 are sequentially formed. An n-pad electrode 26 is formed below the silicon carbide substrate 20, and a p-pad electrode 25 is formed on the transparent electrode 24.
In the conventional nitride semiconductor light emitting device, the transparent electrode lowers a driving voltage of a device by facilitating a current spread, and enhances a quantum efficiency by emitting light generated at a light emitting layer to outside. As the transparent electrode, a metal such as Ni or Au, or a TCO-based oxide such as ITO or IZO are used.
In case of using a metal such as Ni or Au as the transparent electrode, a current spread to the p-GaN layer can be facilitated by lowering an ohmic contact resistance of the p-GaN layer. However, a metal oxide generated at the time of depositing the transparent electrode prevents light generated from a light emitting layer from being emitted outwardly, thereby lowering a light transmittance.
Therefore, in order to increase the light transmittance, a TCO-based oxide such as ITO or IZO is used. However, in case of using the TCO-based oxide, a contact resistance between a P-type nitride semiconductor layer and a TCO electrode is very great thereby to increase a driving voltage.
The transparent electrode can be formed as a layer more than two by using a metal oxide generating metal such as Ni, Pd, Pt, etc. and a current spreading metal such as Au, etc. As the transparent electrode, Ni and Au are mainly used.
For example, a first metal layer is deposited on the p-GaN layer by using the metal oxide generating metal, Ni, and then a second metal layer is deposited on the first metal layer by using a current spreading metal, Au, thereby forming a transparent electrode.
At this time, a metal oxide such as NiO is formed as said Ni is oxidized. The metal oxide supplies a hole to the p-GaN layer.
However, since said metal oxide has an inferior conductivity, a spread of a current supplied from outside to the light emitting layer is prevented. According to this, it is necessary to prevent the metal oxide from being excessively formed.
However, in said general method, that is, in a method for forming a transparent electrode by depositing a metal layer more than two layers on a p-GaN by a separate deposition process, much metal oxide is entirely generated from an interface between the p-GaN layer and the metal layer to the uppest metal layer.
FIG. 3 schematically shows a sectional surface of a transparent electrode formed on a P-type nitride semiconductor in accordance with the conventional art. Referring to FIG. 3, GaN, III–V group compound is formed on a sapphire substrate, then Ni and Au are sequentially deposited on the p-doped GaN layer, and then a thermal annealing is performed to obtain a transparent electrode.
As shown, when the thermal annealing is performed after sequentially depositing Ni and Au, Au of an island shape is formed on the p-doped GaN layer 13 and the Ni is oxidized thereby to form an oxide metal. At this time, the metal oxide supplies a hole to a p-GaN layer, and said Au facilitates a spread of the hole supplied from the metal oxide to the light emitting layer.
FIG. 4 is a view showing a distribution of Au and NiO according to a thickness of a transparent electrode in accordance with the conventional art. In the experiment, a sapphire substrate was used as a substrate, a GaN, III–V group compound was used as a nitride semiconductor, Ni and Au were sequentially deposited on the p-doped GaN layer to form a transparent electrode, and a thickness of the transparent electrode was approximately 1 nm˜100 nm. Also, a thermal annealing for forming the transparent electrode was performed at a temperature of approximately 600° C., and an allowance error range of the temperature in the thermal annealing was approximately ±100° C. At this time, the thermal annealing was performed in an atmosphere that a little amount of oxygen is mixed to nitrogen, and a rapid thermal annealing (RTA) device was used for the thermal annealing.
As shown from the graph, Au is concentrated on the surface of the P-GaN, and NiO is decreased from the surface thereof to the interface between the P-GaN. Also, on the surface of the P-GaN, Au and NiO are similarly distributed.